Sadly, CO2 forcing is not the all or nothing proposition this argument depends on. Try to be patient, make that point and present the current understanding of the 20th century temperature trends.

Answer: No one supporting the theory of Anthropogenic Global Warming claims that CO2 is the only factor in the ocean-atmosphere climate system. It is complex, responsive on many different timescales, and subject to numerous forcings. AGW only claims that CO2 is the primary driver of the current warming trend as seen over the last 100 years. If you look at the temperature record for the 90's you'll notice a sharp drop in '92, '93 and '94. This is the effect of massive amounts of SO2 ejected into the stratosphere by Mount Pinatubo's eruption. That doesn't mean CO2 took a holiday, it was just temporarily overwhelmed by another opposite forcing."